This invention relates to a fluid operated hammer used in drill bore holes and the like.
Throughout this specification, the term "upper" when used in relation to a hammer shall refer to that end of the hammer adjacent the top sub and the term "lower" when used in relation to a hammer shall relate to that end of the hammer adjacent the drill bit.
In fluid operated hammers currently in use, fluid is supplied to the hammer to cause reciprocation of the piston through ports and passageways formed in the casing. As a result of wear in the casing caused by the reciprocation of the piston within the casing such hammers rapidly lose their efficiency due to the increase in the area of the annulus separating the piston and the casing. In addition, the manufacture of such hammers is difficult due to the necessity to provide the passageways through the walls of the casing. Furthermore, due to the presence of the passageways in the walls of the casing the internal cross-section area available within the casing for the piston is reduced, as a result the pressure area and the mass of the piston is reduced and the potential effectiveness of the hammer is reduced.
It is an object of this invention to provide a fluid operated hammer not requiring passageways in the walls of the casing for delivery of fluid to the piston.
In other fluid operated hammers which utilise an axial feed tube to apply fluid to both the drill bit and the reciprocating piston the supply of fluid to effect reciprocation of the piston is for a single relatively short period at either end of the travel of the piston. As a result the downward thrust exerted on the piston is produced by gravity when the hammer is used for holes extending downwardly and the pressure applied by the fluid injected into the space above the piston when it is at its uppermost position.
It is an object of this invention to provide for the injection of fluid into the space above the piston for a significant proportion of the degree downward movement of the piston within the hammer.
In one form the invention resides in a fluid operated hammer comprising; a casing; a top sub mounted to one end of the casing; a drill bit mounted to the other end of the casing; a feed tube located concentrically within the casing and extending for a portion of the length of the casing from said one end; and a piston slidably mounted in the casing for longitudinal movement between the top sub and drill bit over said feed tube; said feed tube being formed with at least one elongate aperture wherein the longitudinal dimensions of said at least one aperture correspond to a significant proportion of the degree of reciprocation of said piston; a first passageway provided in the piston and extending between the central bore and the external face thereof to provide a communication between the said at least one aperture and the space between the piston and the top sub when the piston is at or near the top sub and for a significant proportion of the degree of movement of the piston from the top sub to a drive the piston towards the other end of the casing; a rebate formed in the internal face of the casing adjacent or near the other end; and a second passageway in the piston and extending from the central bore thereof to the external face thereof to provide communication between said at least one aperture and said rebate when said piston is abutting said drill bit to admit fluid into the space between the piston and drill bit and drive the piston towards the one end.
According to a preferred feature of the invention the feed tube is mounted to and located in the one end of the casing bore and is isolated from the top sub to ensure that the piston and feed tube are concentric to one another within the casing.
According to a further preferred feature the at least one aperture comprises a set of longitudinally spaced apertures.
According to another preferred feature of the invention the at least one aperture comprises an axial elongate slot formed in the wall of the feed tube.
According to a further preferred feature the internal face of the casing and the external face of the piston adjacent the top sub are each formed with an annular rib wherein said ribs engage each other at an intermediate position in the degree of travel of the piston to close communication via said first port between the at least one aperture and the space between the top sub and the adjacent end of the piston.
According to a preferred feature of the last mentioned feature the annular rib in the casing is provided by a sleeve located with the casing.
According to a further preferred feature, the feed tube can be replaced with another of differing characteristics to vary the periodicity of communication of fluids between the ends of the piston.
According to a further preferred feature of the invention the feed tube is mounted in the casing via a spacer and centralising ring wherein said ring is capable of being replaced by a ring of differing dimensions to vary the volume of the chamber between the piston and the top sub.
According to another preferred feature the drill bit is capable of limited longitudinal slidable movement in the casing and is retained in the casing via a bit retaining ring located on the internal face of the casing and a rib on the external face of the drill bit, said ribs being engaged when the bit occupies the extended position in the casing and wherein an annular member of resilient shock absorbing material is incorporated in said bit retaining ring to absorb some of the shocks imposed thereon by the bit.
In existing hammers, it has been known for such hammers to prematurely fail due to excessive wear of the casing and/or feed tube and/or piston. One major cause of such excessive wear has been the misalignment of the feed tube within the bore of the casing. It is current practice in at least one form of hammer to mount the feed tube of the hammer into the top sub of the hammer which is then threadably mounted into the casing. Alternatively it is common practice to locate the feed tube in a counter-bored portion at one end of the casing. In such circumstances, to ensure that the feed tube is aligned concentrically within the bore of the casing on assembly of the hammer, involves precise machining of the feed tube, casing, counter-bore, casing thread, top sub and top sub thread and ring stringent quality control.
It is an object of this invention to reduce the degree of precise machining of components of fluid operated hammers.
In another form the invention resides in a fluid operated hammer comprising a casing having a drill bit mounted to one end and a top sub mounted to the other end, a feed tube mounted concentrically within the casing and a piston longitudinally slidable within the casing between the drill bit and the top sub over the feed tube, wherein the feed tube is mounted and located in the one end of the casing and is isolated from the top sub.